1. Field of the Invention
This invention relates to variable focal length objectives, and more particularly to such objective which has a plurality of lens units with the separation between the first and second lens units being varied to vary the focal length of the entire system.
2. Description of the Related Art
The aberration problem of the variable focal length objective, especially the zoom lens that varies its focal length continuously, must be solved, besides the good correction of aberrations in the standard setting, for as far minimized variation with zooming of the aberrations as possible. To achieve this, all the lens units are required to be at least individually corrected well for spherical aberration, coma and astigmatism in each unit. It has, therefore, been the prior art practice that each lens unit is made up by using several lens elements.
Because the total sum of the lens elements of all the units became enormous, the weight and size of the entire system were very heavy and very long in the longitudinal direction, and the assembly and adjustment of the lens elements in each unit was very difficult to perform with high accuracy in respect to the axial alignment and the spacing.
Recently, much interest has aroused in developing a variable focal length objective of more compact form with an increased range of magnifications. Yet, from the obstacle of the above-described problem of aberration correction it was also very difficult to achieve it by relying on the method of trimming some lenses from each lens unit.
For example, in the type of zoom lens comprising a plurality of lens units of which the first and second counting from the front are respectively positive and negative in power and the separation between the first and second lens units, its bulk and size can be reduced, speaking in the concept of Gauss theory, either by strengthening the power of each lens unit, or by shortening the intervals between the principal points of the successive two of the lens units. As to extend the zooming range, the method of strengthening the power of each lens unit may also be employed. There is another method of increasing the total zooming movement. When the requirements of reducing the size of the zoom lens of the type described above and of increasing the zoom ratio are to be fulfilled simultaneously, it is certainly natural in the concept of Gauss theory to choose the method of increasing the power of each lens unit. In an actual lens system, however, the increase in the power of the lens unit calls for an increase in the number of constituent lens elements in order to achieve good stability of aberration correction over the entire zooming range. This may otherwise be achieved by increasing the power per one lens element with its surface curvatures being strengthened. To allow for the minimum acceptable edge thickness of every lens element, the central thickness of the positive lens and the axial air separation between the adjacent concave surfaces are caused to become very large. In either case, therefore, the physical length of each of the lens units increases. Also since the principal point interval has to be increased, the optical total length of the entire system can no longer be shortened. Such an increase of the length of the lens unit gives rise to another problem that as the space in which the lens unit movable for zooming makes excursion becomes small, a desired increase of the zoom ratio can no longer be achieved.
Further with the first or second lens unit having an increased overall thickness due to the increase of the number of constituent elements, to admit of the oblique pencile, the diameter of the first or front lens unit must be increased. Thus, the attempt to advance the compactness was frustrated. Therefore, the use of any of the prior known methods, because of effecting the result of such a vicious cycle, could not filfill both requirements of achieving a much-desired reduction of the bulk and size of the zoom lens and of achieving a much desired increase of the zoom ratio while preserving good stability of aberration correction throughout the entire zooming range.